Because of their good dimensional stability, weather resistance, mechanical properties and durability, synthetic fibers especially polyester fibers, polyamide fibers, etc. are heretofore indispensable for the materials for clothing, interior decoration, etc. Depending on their use, however, the fibers are required to have some additional specific functions. For example, for the fibers in the field of interior decoration, especially in the field of artificial leather for the facing materials for seats for railroad coaches, seats for automobiles, seats for airplanes, etc., it is extremely important to be resistant to flames.
Heretofore, a substrate of nonwoven fabric that has a binder of a polymer elastomer in the interspaces of entangled fibers therein is used for the substrate layer for artificial leather. When the surface of the substrate is coated with a resin layer, it gives grain-leather-like artificial leather; and when napped, it gives suede-like artificial leather. For making the artificial leather substrate layer resistant to flames, generally employed are a method of adhering a flame retardant to the surfaces of the fibers and the binder that constitute the substrate layer, through post-treatment or the like; a method of lining the back of the substrate layer with a flame-retardant sheet; a method of using fibers spun from a thermoplastic polymer kneaded with flame-retardant particles thereinto, etc.
Of those methods, the method comprising such post-treatment is problematic in that the feel of the artificial leather produced is not good and, especially when the artificial leather is a suede-like one having a napped surface, its dense naps pill through the post-treatment for flame retardancy and the surface pills worsen the appearance of the suede-like artificial leather. On the other hand, the method of lining the back of the substrate layer with a flame-retardant sheet is also problematic in that there often occurs a difference of the flame retardancy between the face and the back and the feel of the artificial leather thus produced is not good.
One concrete embodiment generally employed for the method of kneading a thermoplastic polymer for fibers with a flame retardant thereinto comprises kneading a flame retardant having an active ingredient of a phosphorus or halogen compound into a shaping material such as polyethylene, polypropylene, polyethylene-polypropylene copolymer, polystyrene, etc., to thereby make the resulting polymer fibers resistant to flames. On the other hand, the method of kneading the flame retardant into a polyamide-type polymer such as nylon 6, nylon 66, nylon 610, etc., or into a polyester-type polymer such as polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, etc. is problematic in that the spinning temperature must be specifically controlled and the polymer and the flame retardant to be used must be specifically selected from the viewpoint of the stability of the flame retardant and the polymer at the melt-spinning temperature and the method gives low productivity.
Still another problem with the method of kneading a flame retardant to the fibers is that the method is applicable to flame-retardant fibers having a regular fineness, or that is those thicker than 0.5 dtex in fineness, but could not to superfine fibers. In this connection, for example, for suede-like artificial leather for which the fineness of the fibers is a matter of importance, the fineness of the fibers constituting it is preferably at most 0.5 dtex for raising a dense and high-quality fibrous nap on it and for making it have a good feel, and for further making it have a natural leather-like excellent appearance. However, when flame-retardant particles are kneaded into such superfine fibers, then the physical properties of the fibers greatly worsen because of the relationship between the particle size of the flame-retardant particles and the area of the cross section of each fiber, and, as a result, the resulting artificial leather made of such fibers could be no more practicable.
Even when a flame-retardant organic substance or the like could be dispersed in such superfine fibers not worsening the physical properties of the fibers, the flame-retardant organic substance will drop off from the fibers in the subsequent wet treatment of forming the binder into a porous substrate through solvent removal for coagulation; or for a sheet of superfine fibers-forming fibers, especially those having a sea-island structure, the flame-retardant organic substance will also drop off from the fibers in the step of removing the sea component from the fibers that is generally employed in forming the intended superfine fibers. In most cases, therefore, the substrate or the sheet produced could not attain the intended flame retardancy level.
Even when superfine fibers are produced through direct spinning or the like and when an organic flame retardant is applied thereto, the flame retardant will bleed out on the surfaces of the fibers. If so, while the products of the fibers are used, the flame retardant will also bleed out on their surfaces. In particular, when they are used for seats for vehicles, the flame retardant thus having bled out on their surfaces will vaporize to often cause a problem of windshield or windowpane fogging, etc.
On the other hand, the flame retardant that may be dispersed in a polymer elastomer in producing a flame-retardant leather-like sheet substrate may be any known flame retardant for ordinary resins including, for example, halogen, phosphorus or nitrogen-containing organic flame retardants, and inorganic compounds such as metal hydroxides, red phosphorus, silicon compounds, etc. Anyhow, the requirements for the flame retardant are that it does not promote the degradation of the polymer elastomer and the superfine fibers to which it is applied, and it does not substantially dissolve and decompose in the coagulation bath to be used in producing the intended leather-like sheets and also in the processing solutions to be used in the step of forming the superfine fibers, etc. On the other hand, in case where the leather-like sheets produced contain a halogen substance, they release harmful substances such as dioxins, etc. when they are fired, and will give some load to the environmental problem.
The object of the invention is to provide a halogen-free and durable flame-retardant leather-like sheet substrate having a soft feel, which is produced by imparting flame retardancy to superfine fibers having a single-fiber fineness of at most 0.5 dtex, preferably those obtained through removal of at least one component from conjugate or mixed spun fibers of at least two or more components of thermoplastic polymers, not so much worsening the properties of the resulting superfine fibers, and by imparting flame retardancy also to the polymer elastomer to be in the substrate not promoting the degradation of the polymer elastomer.